1. Field of the Invention
The field of art to which this invention pertains includes pressure regulating valve means for use in fluid operated control systems. More particularly, the regulating valves of this invention are constructed to control and regulate fluid pressure, and they may be disposed, for example, between a source of pressurized fluid and a device operated thereby, such as a transmission clutch.
2. Description of the Prior Art
Briefly, this invention constitutes an improvement in pressure regulating valves, with such valves often being used in the prior art for regulating pressure to a fluid operated clutch of the type often found in the constant-mesh power-shift transmissions of industrial vehicles, such as lift trucks, and other machines. Specifically, such prior art control valves are frequently constructed to permit an immediate relief of a portion of the clutch application pressure followed by a regulated decrease or increase in the fluid pressure applied to the fluid clutch whereby the latter may be gradually disengaged or engaged by the operator. One particular use of this type of regulating valve, contemplates the control of a fluid-pressure actuated clutch for the transmission of an industrial vehicle, such as a lift truck, where it is often necessary to hold a truck stationary while the lift fork is operated at a speed corresponding to full engine speed without shifting the transmission to the neutral position. It is desirable, in such circumstances, to be able to precisely move or "inch" the vehicle in either the forward or reverse direction in order to achieved accurate positioning in the handling of materials.
Various controls and regulating valves for performing the abovestated function in conjunction with power-shift transmissions employing fluid pressure actuated transmission clutches have been disclosed previously as exemplified by U.S. Pat. No. 3,143,127, to Frost, issued Aug. 4, 1964. In order to carry out its regulating valve function, the valve means, such as described in said patent, have been provided to effect improved gradual engagement and disengagement of the vehicle clutch and the relief of excess pressure in the system by operating such valve means in conjunction with the application and release of the vehicle brakes. In such structures, wherein the operation of the valve means may be either hydraulic or mechanical, when the brakes are actuated the valve means shifts to a position partially interrupting fluid flow from a clutch control valve and affects controlled partial deactuation of the clutch means by maintaining a relatively low fluid pressure, thus providing control of the torque to the wheels of the vehicle.
Prior art patents dealing with inching valves wherein the flow of pressurized fluid to the clutch undergoing inching is completely cut off during at least a portion of the inching cycle include U.S. Pat. Nos. 2,861,480 to Curtis and 3,339,672 to Crandall.
The presence of a bypass arrangement, which keeps at least some fluid flowing to the clutch undergoing inching, insures a continuous supply of fluid to the clutch so that, at the termination of the inching function, there is no delay occasioned by the refilling of the clutch. Prior art U.S. Pat. Nos. 2,814,371 to Bolster et al. and 2,972,906 to Schroeder, disclose inching valves that incorporate internal bypass arrangements.